1. Field of the Invention
The present invention relates to non-reciprocal circuit elements, and more particularly, to a non-reciprocal circuit element, such as an isolator or a circulator, used in a microwave band.
2. Description of the Related Art
Non-reciprocal circuit elements, such as isolators and circulators, have characteristics of transmitting signals only in a certain predetermined direction and not in the opposite direction. These characteristics of isolators, for example, are used in transmitter circuits of mobile communication devices, such as automobile phones and mobile phones.
Japanese Patents Nos. 4155342 and 4197032 describe two-port isolators used as non-reciprocal circuit elements of the above-described type. The two-port isolators have the basic structure in which a first center electrode and a second center electrode are arranged on a surface of a ferrite body so as to cross each other in an insulated state. A resistor is connected between an end of the first center electrode that is connected to an input port and an end of a second center electrode that is connected to an output port. The resistor is connected in series with an inductor. In each of the two-port isolators, insertion loss is reduced and isolation characteristics are improved.
Recently, it has become possible for a single mobile phone to provide communication in a plurality of frequency bands. To achieve this, according to the related art, one isolator is used for each frequency band. However, in this case, the number of components is increased. Therefore, there is a demand for non-reciprocal circuit elements that can be used in a plurality of frequency bands. In other words, there is a demand for non-reciprocal circuit elements having a single input port and at least two output ports which can be used in a plurality of frequency bands.
To achieve operations in a plurality of frequency bands, the inventors of the present invention have devised a non-reciprocal circuit element in which a pair of two-port isolators similar to those described in Japanese Patents Nos. 4155342 and 4197032 are combined. The two-port isolators are of the high-pass type. Referring to FIG. 14, if the isolators are combined so as to operate in frequency bands f1 and f2, a harmonic frequency band of the frequency band f2 overlaps the frequency band f1. This causes a communication failure.
Japanese Unexamined Patent Application Publication No. 9-93004 describes a three-port isolator with an increased operation frequency band. According to this publication, a non-reciprocal circuit element includes first and second non-reciprocal circuits that operate at first and second frequencies, respectively, and first and second phase converter circuits that are connected to input and output ports of the first and second non-reciprocal circuits, respectively, and that are substantially opened at the second and first frequencies, respectively. The first and second phase converter circuits and the first and second non-reciprocal circuits are connected in parallel to form a single circuit element unit.
The three-port isolator described in Japanese Unexamined Patent Application Publication No. 9-93004 is basically of a low-pass type, and there is no risk of interference between the first and second frequencies since the second phase converter circuit is provided. However, when four phase converter circuits are provided to increase the operation frequency band, the insertion loss increases. In addition, the number of components also increases, which makes it difficult to achieve reduction in size.